1. Field of the Invention
The present invention is directed to a computer tomography apparatus of the type which undertakes a helical scan of an examination subject.
2. Description of the Prior Art
It is known in conventional computer tomography systems to produce a conventional x-ray image (shadowgraph) of an examination subject, for the purpose of defining the examination region before beginning the computer tomographic examination. For this purpose, the patient is usually moved through the measurement opening, with the x-ray focus being held at a fixed angular position, and a conventional shadowgraph, i.e., an x-ray projection image, is produced continuously, or pulsed line-by-line. This known technique of obtaining a conventional x-ray image using a computer tomography apparatus has several disadvantages. First, in addition to offering the standard computer tomographic exposure mode with continuous scanning, a further exposure mode must be offered wherein the x-ray focus is held stationary. Additionally, as a result of the creation of two separate exposures, the time required for the complete examination is higher by several minutes compared to the time needed for the actual production of the computer tomogram. Such lengthening of the exposure time is not only economically undesirable, but also the patient frequently changes position between the two exposures, so that a shifting between the selected body region and the body region which is actually registered during the examination can occur.